Traditionally, results from assays involving luminescent or radioactive labels would be obtained by exposing a blot, membrane, dried gel, etc. to X-ray film. After exposure, the film would be developed so the presence, absence, or intensity of signal from the assay could be determined. Film is convenient for recording results, provides crisp signal, and comparative results. Film results are not, however, quantitative. Moreover, a strong or weak signal can lead to overexposure or underexposure, respectively, so that the exposure must be repeated to obtain useful results. When the assay relies on a chemiluminescent signal that decays with time, it might not be possible to obtain results from a second or third exposure.
Digital imaging, e.g., with a CMOS (complementary metal oxide semiconductor) or CCD (charge coupled device) scanner, allows the user to obtain quantitative results. In some cases however, the result can be distorted or difficult to view, e.g., because of pixel size.